During the past several decades, coronary heart disease has come to occupy the first position among the causes of death in the developed areas of the world. In the United States, for example, this disease is responsible for over one-half million deaths yearly. And of this number, more than half occur suddenly, outside the hospital, and therefore before the patient is able to obtain the necessary medical assistance. Although the precise cause of sudden death in coronary heart disease has not yet been entirely clarified, the available evidence permits the medical field to ascribe death in the majority of these cases to grave disturbances in cardiac electrical activity culminating in ventricular fibrillation.
Another frustrating but related problem is the present inability to deal effectively with lethal and nonlethal arrhythmias outside of a hospital setting. Within the hospital environment, however, recent experience has clearly demonstrated that ventricular fibrillation and its frequent precursor, ventricular tachycardia, are reversible phenomena when prompt cardioversion of the heart is instituted. Under such circumstances, cardiac function can frequently be restored to normal without the patient suffering from residual disability. Unfortunately, however, the present state of the art makes cardioversion very dependent upon a highly specialized medical environment, thus limiting such treatment to fully equipped, modern hospitals.
There is no question that a great need exists for a defibrillator which would be carried by those who are prone to having one of the many life-threatening arrhythmias generally discussed above. Thus, in some patients having coronary heart disease, a fatal outcome from ventricular tachycardia or ventricular fibrillation could be avoided, even in the absence of immediate medical assistance. The first step, of course, is the detection of those prone to suffering from cardiac malfunctions leading to ventricular tachycardia or ventricular fibrillation.
While it is not possible to predict with unerring exactness which patients suffering from coronary heart disease will die from ventricular fibrillation or ventricular tachycardia, several high risk groups of patients can be recognized. For example, patients who have experienced myocardial infarction, even though they may be surviving in good health, run a substantial risk of dying suddenly, a risk several times greater than that associated with the general population. Further, if patients with myocardial infarction have a history of serious ventricular arrhythmias and/or of cardiac arrest, or if evidence of persistent myocardial irritability is present, it may be logically assumed that the risk of sudden death is increased substantially. A patient like those described above would greatly benefit from an automatic defibrillator.
Also, such an automatic defibrillator would be an asset to those patients who have suffered myocardial infarction in the coronary care unit and remain hospitalized in the coronary care unit or some other area of the hospital. Under such circumstances, the defibrillator could be used temporarily for the remainder of the expected hospital stay; or the automatic defibrillator could be permanently implanted for use both in the hospital and after discharge. And another recognizable class of patients particularly in need of an automatic defibrillator is the class composed of those who have not shown prior histories of myocardial infarction but who show severe symptoms of coronary heart disease, such as ventricular arrhythmias resistant to medical treatment or angina pectoris.
From the brief discussion above, there should be little doubt that the possible applications for an automatic defibrillator are numerous. Such an automatic defibrillator has been developed by Medtronic, Inc. and is described in U.S. Pat. application Ser. No. 124,326, filed Mar. 15, 1971, now abandoned by Mieczyslaw Mirowski, et al. and entitled "CARDIOVERTER HAVING SINGLE INTRAVASCULAR CATHETER ELECTRODE SYSTEM."
The automatic standby defibrillator described in the above-identified patent application employs a pressure sensing element attached to a body implantable electrical lead such that it can be positioned within the right ventricle of the heart. Since the pressure in the heart drops severely when the heart goes into the fibrillation state, ventricular fibrillation can be easily detected by monitoring heart pressure. However, several difficulties with measuring heart pressure are encountered. One disadvantage with using pressure as an indicator of the fibrillation state is that the small pressure sensing elements which are suitable for use with body implantable electrical leads are quite expensive. A second disadvantage with using these pressure sensing elements is that they must either be located alongside, on the outer surface, or at the tip of the body implantable electrical lead to obtain accurate pressure readings. Locating the detection means alongside the catheter tends to make the catheter bulky and inflexible. Fibrotic tissue tends to build-up around the sensing element when it is positioned within the heart. This build-up tends to dampen the pressure transducer mechanism, thus giving inaccurate pressure readings. Inaccurate pressure readings may also result when the element is located at the tip and wedged into the apex of the right heart ventricle as the transducer will then, of course, tend to be dampened by the surrounding heart muscle.
The apparatus of this invention uses a single intravascular electrode of the type described in U.S. Pat. application Ser. No. 202,238, filed Nov. 26, 1971, by Rollin H. Denniston, III, entitled "MUSCLE CONTRACTION DETECTION APPARATUS," to perform three functions: namely, (1) detecting heart contractions; (2) detecting heart electrical activity in the form of R waves; and (3) applying electrical impulses to the heart for cardioverting it.
Thus the apparatus of this invention overcomes many difficulties existent in the prior art devices while providing a compact and practical automatic cardioverting system.